1. Field of the Invention
The present invention relates to an X-ray mask used in transferring an image to a wafer by exposing the mask with an X-ray through the mask having a pattern of the image. The present invention is also applied to masks for an electron-beam cell projection lithography and for an ionbeam lithography as well as to the x-ray mask.
2. Description of the Related Arts
FIG. 1 is a cross sectional view of a conventional X-ray mask. The X-ray mask comprises a main substrate 1A which is formed by forming X-ray transparent thin films 2A and 2B on the top and bottom surfaces of a mask substrate 1, a thin membrane 4 having a thickness of about 2 micron which can be obtained by eliminating the bottom of the mask substrate 1 in order to improve the optical transparentness, an absorber 3 for effectively interrupting the transmission of the X-ray by absorbing the X-ray, and a support ring 5 attached to the bottom of the main substrate 1A for mechanical stability and convenient handling. The membrane 4, which is a chip site region where designed device patterns are located, has a micropatterned absorber of 0.15 micron line width. Accordingly, it is required to make minimum the stress distortion and the thermal distortion of the membrane, and to improve the flatness of the membrane. In the measurement of the flatness and the distortion, parameters of OPD (Out-of-Plane Distortion) and IPD (In-Plane Distortion) are typically used. These parameters are greatly influenced by each processing step such as: the formation of the membrane through the deposition of the Xray transparent thin film and back-etching of the mask substrate; the attachment of the support ring; the micropatterning of the absorber onto the membrane; and a mounting of the final mask to an X-ray stepper.
FIG. 2 is a back plan view of the conventional X-ray mask. The X-ray mask can be obtained by depositing an Xray transparent thin film on the mask substrate-1 and by attaching the support ring 5 to the back side of the main substrate 1A. However, because the thermal coefficient of expansions TCES of the support ring 5 and of the main substrate are different, if the mask experiences any temperature variation after being attached to the support ring, the main substrate may be distorted in a convex manner as shown in FIG. 3A or in a concave manner as shown in FIG. 3B. The distortion of the main substrate significantly degrades a pattern position accuracy and an overlay accuracy. Therefore, it is very important to properly select the material and the structure of the mask substrate, the X-ray transparent thin film and the support ring.
As an instance, the thermal coefficient of expansion of a silicon wafer typically used as the mask substrate is 2.6.times.10.sup.-6 /.degree.C., and a pyrex used as the support ring has a TCE of 3.6 10.sup.-6 /.degree.C. greater than the mask substrate. Accordingly, when the support ring is attached to the back surface of the main substrate by using an anodic boding carried out in a temperature of approximately 300.degree. C., and the assembly is cooled in room temperature, thermal from the difference of the TCEs is very great.
As exclaimed before, the distortion from the difference of the TCE is inevitable as long as the interfaces between physically different materials such as the mask substrate and the support ring exist. If the silicon wafer typically used as the mask substrate is used as the support ring, the thermal distortion can be enhanced but its mechanical strength is not enough to support the substrate.